1. Field of the Invention
This invention relates to reducing methane production and increasing liquid (C.sub.5.sup.+) yields in Fischer-Tropsch hydrocarbon synthesis reactions by recycling olefins separated from the product directly into the reactor bed. More particularly, this invention relates to reducing methane production in catalytic Fischer-Tropsch reactions wherein hydrocarbons are synthesized from a feed comprising a mixture of CO and H.sub.2 and adding one or more olefins directly to the reactor bed, wherein the olefin is separated from the product stream or obtained from an independent source and then added directly to the reactor bed.
2. Background of the Invention
The production of hydrocarbons from mixtures of H.sub.2 and CO via the Fischer-Tropsch process is well known to those skilled in the art. As opposed to the well known "methanation" process, which produces methane as synthetic natural gas from mixtures of H.sub.2 and CO, the Fischer-Tropsch process is more generally aimed at producing higher value products, such as chemical feedstocks and liquid fuels. Thus, high methane make is undesirable in Fischer-Tropsch synthesis processes because it is a relatively low value product which is formed at the expense of more desirable products. It is also uneconomical to try to convert the so-formed methane back into a CO and H.sub.2 mixture and recycle it back into the reactor.
Methane make in Fischer-Tropsch reactions is often expressed by a term known as methane selectivity and also carbon selectivity (% of CO converted which appears as methane) which is almost identical to weight percent. Methane selectivity can be defined by either of two methods, which are: (a) mole percent CH.sub.4 produced based on the amount of CO consumed; or (b) weight percent of CH.sub.4 produced based on total hydrocarbon products formed.
Many different catalysts and processes have been disclosed for Fischer-Tropsch synthesis, some of which have extremely high methane make. Thus, U.S. Pat. No. 4,077,995 discloses synthesis of C.sub.1 -C.sub.4 aliphatic hydrocarbons over a catalyst comprising a sulfided mixture of CoO, Al.sub.2 O.sub.3 and ZnO, while U.S. Pat. No. 4,039,302 discloses C.sub.1 -C.sub.3 hydrocarbon production using a mixture of the oxides of Co, Al, Zn and Mo. U.S. Pat. No. 4,151,190 discloses C.sub.2 -C.sub.4 hydrocarbons from mixtures of CO and H.sub.2 using a supported catalyst comprising a metal oxide or sulfide of Mo, W, Re, Ru, Ni or Pt, plus an alkali or alkaline earth metal, with Mo-K on carbon being preferred. U.S. Pat. Nos. 4,243,553 and 4,243,554 disclose MoS.sub.2 as a Fischer-Tropsch catalyst. Many other catalysts are known to be useful for Fischer-Tropsch synthesis employing metals such as iron, copper, titania, etc. These are known to those skilled in the art.
The type of catalyst used and process conditions employed have an important bearing on CH.sub.4 selectivity. For example, nickel gives a high CH.sub.4 selectivity and is used mainly as a methanation catalyst. Methane selectivity usually increases with increasing temperature, decreasing pressure and increasing the H.sub.2 /CO ratio of the feed. Accordingly, process conditions are selected so as to minimize CH.sub.4 selectivity and C.sub.2 -C.sub.4 selectivity, also while maintaining a relatively high reaction rate, as is well known to those skilled in the art.
It is known that CH.sub.4 selectivity is influenced by the choice of promoter and support, such as alkali metal promoters reducing CH.sub.4 selectivities of iron catalysts. It is also known in the art that noble metals, such as ruthenium, supported on inorganic refractory oxide supports exhibit superior hydrocarbon synthesis characteristics with relatively low methane production. Thus, U.S. Pat. No. 4,088,671 suggests minimizing methane production by using a small amount of Ru on a cobalt catalyst. Examples of supported ruthenium catalysts suitable for hydrocarbon synthesis via Fischer-Tropsch reactions are disclosed in U.S. Pat. Nos. 4,042,614 and 4,171,320, the disclosures of which are incorporated herein by reference. It is also known that the type of support used also influences methane production. In the case of supported ruthenium catalysts the use of a titania or titania-containing support will result in lower methane production than, for example, a silica, alumina or manganese oxide support.
The present invention differs from U.S. Ser. No. 563,109, filed on Dec. 19, 1983, in that in U.S. Ser. No. 563,109 alpha olefin was added directly to the feed of the CO/H.sub.2, whereas in the instant invention the alpha olefin, which is recycled from the product stream or obtained from a separate, independent source, is added directly into the reactor bed through an inlet port in the side of the tubular reactor, wherein the inlet port is positioned below a point which is 10% of the distance from the top of the reactor bed to the bottom of the reactor bed and above a point which is 10% of the distance from the bottom of the reactor bed to the top of the reactor bed.
Those skilled in the art recognize the need for reducing methane production still further, even when employing catalysts comprising ruthenium supported on titania.